import React, { useEffect, useState } from 'react'

export default function App() {
let [opacity,setOpacity]=useState(1)
// 初始化的时候添加一个定时器
useEffect(()=>{
  const opacityTime=setInterval(()=>{
    opacity-=0.1
    if(opacity<=0){
      opacity=1
    }
    setOpacity(opacity)
  },100)

  return ()=>{
    clearInterval(opacityTime)
  }
},[])

  return (
    <div>
      <h1 style={{opacity}}>ReactHook真好用</h1>
    </div>
  )
}
